Power amplifier klystrons operating in wide frequency bands

ABSTRACT

Klystrons of the invention comprise resonant cavities tuned to frequencies higher than the maximum frequency of the frequency band B of operating in order to improve the efficiency at frequencies higher than the centre frequency Fo of the band B, and harmonic cavities tuned to frequencies slightly below the frequency 2 Fo - B, in order to improve the efficiency at frequencies of less than Fo. Wide-band improved klystrons of this kind can be used for operation within wide power ranges and even at relatively low powers, for example some few hundreds of killowatts.

United States Patent Faillon et a1.

11] -3,775,635 [451 Nov. 27, 1973 1 POWER AMPLIFIER KLYSTRONS OPERATINGIN WIDE FREQUENCY BANDS [75] Inventors: Georges Faillon; Grard Firmain,

both of Paris, France [73] Assignee: Thomson-CS1", Paris, France [22]Filed: Aug. 17, 1972 [21] Appl. No.: 281,272

[30] Foreign Application Priority Data Sept. 16, 1971 France 7133392[52] U.S. Cl 3l5/5.43, 3l5/5.39, 315/5.52 [51] Int. Cl. l-l0lj 25/10[58] Field of Search 3l5/5.39, 5.43, 5.52

[56] References Cited UNITED STATES PATENTS 3,594,606 7/1971 Lien315/5.43

3,622,834 11/1971 Lien 315/5.52 3,483,420 12/1969 Lien et al....3l5/5.52 X 3,453,483 7/1969 Leidigh 315/5.39

Watson et al 3l5/5.43

2,591,910 4/1952 Braford 2,579,480 12/1951 Feenberg 315/5.43 3,249,7945/1966 Staprans et a1. 3.l5/5.43

Primary Examiner-Rudolph V. Rolinec Assistant Examiner-Saxfield Chatmon,Jr. Att0meyCushman, Darby et a1.

[5 7 ABSTRACT Wide-band improved klystrons of this kind can be used foroperation within wide power ranges and even at relatively low powers,for example some few hundreds of killowatts.

2 Claims, 4 Drawing Figures POWER AMPLIFIER KLYSTRONS OPERATING IN WIDEFREQUENCY BANDS The present invention relates to improvements inwidebandklystron power amplifiers. It relates more particularly toklystrons improved to give them high efficiency throughout the whole ofthe working frequency band, this even in the case of relatively lowpowerklystrons producing outputs in the order of some few hundreds ofkilowatts for example.

It is well known that the combination of conditions of this kind, namelywide passband and relatively low power, makes it extremely difficult toproduce klystrons which have satisfactory efficiency throughout theband.

In other words, although it is relatively straightforwardto improve theefiiciency of a narrow-band'klystron by utilising harmonic cavitiestuned substantially to twice the centre frequency of the working band,it is equally well known that this method leads to anefficiency/frequency curve which is more peakedand only producessatisfactory efficiency in a relatively narrow band; there is noquestion of utilising this artifice for wide-band klystrons.

Generally speaking, to improve the efficiency of wide-band klystrons,another method is used; this consists in optimising the efficiency atthe centre of thepassband by tuning the last two or three-cavities-ofthat section of the klystron often referred to as the amplifier"section, to frequencies which are higher than the operating frequencies.

This amplifier section, also often known as the large signal section,comprises a certain numberof cavities where the effective high-frequencypower (HF) is generated.

It is generally preceded by a' pre-amplifier" or small signal section,to the input of which the HF signal for amplification is applied andwhich comprises a certain number of cavities where the gain isbuilt up;a judicious choice of the number of these cavities, their arrangementand their tuned frequencies which are staggered in relation to oneanother in accordance with a known technique, makes it possible toachieve a suitable gain'and passband in this pre-amplifier section.

This section is followed by a circuit for picking off' the amplified HFpower, which in the conventional way and depending upon the type ofklystron, comprises one or more cavities, possibly followed by filters,a resonant spiral, etc.

Although this optimising of the band centre efficiency leads to verysmall variations in the band as a whole, talking at any rate of veryhigh-power klystrons producing for example some megawatts, thisunfortunately not the case in klystrons of lower power, producing forexample some few kilowatts or even some few hundreds of kilowatts.

At these relatively lower powers, the procedure outlined here leads toan efficiency curve which peaks at the centre of the passband anddecreases very rapidly as the frequency progresses to either side of thecentre frequency. It is difficult if not impossible to obtain klystronswhich will operate properly in wide passbands, that is to say oneshaving widths in the order of percent of the centre frequency in band Sfor example.

The object of the present invention is to produce klystron amplifierswhich will operate properly and in particular with a high andsubstantially constant efficiency,

quency (or frequencies) which is/are higher than the maximum frequency Fof said frequency band'Band such that the frequency of thehigh=frequencysignal,.

within wide frequency bands, even at relatively low output powers.

In accordance with the invention; a klystron; for

power amplification of a highfrequency signal; which: klystron can beoperated'within a-wide frequency band" B centred on a frequency F 0comprises, within an evac-' uated enclosure, an electron-gunemittinganelectron beam, a first set of resonant cavities known'as thepreamplifier section, a second set of resonant cavities known as theamplifier" section, a high frequency cir cuit for picking off theamplified-high frequency signal;

anda collector, one/several of the resonant cavity/ies in the saidamplifier" section'beingtunedto a frefor which the klystron has amaximum efficiency is higher than the centre frequency F0 and lower thansaid'maximum frequency F and said amplifier section furthermorecomprising one/several harmonic cavity/ies tuned to a frequency 2Fo Btwice the-minimum frequency of the'band B and less than said'2Fo-Bfrequency by an amount such'that an increase inthe klystron efficiencyis achieved by this/these'cavity/ies in respect of high frequencysignal'shavinga frequency less than the centre frequency and greaterthan the minimum frequency F 1 of the frequency band B.

This novel combination of cavities tuned specially andin particular insuch a fashionthat each of them promotes efficient operation of theklystron not at the centre F0 of the band Bas was the case hitherto, butat each side thereof, makes it possible to achieve'high efficiencieswhich vary very little throughout wide frequency bands.

Other objects, features and results of the inventionwill become apparentfrom the ensuing description, given by way of non-limitative exampleandillustrated by the attached figures where FIGS. 1 and 2 are graphsindicatingthe'behaviour. of the variation, as a function of frequency,in the efficiency of wide-band conventional klystrons for high (FIG. I)and low (FIG. 2) power'2) power outputs FIG. 3 is a graph indicating thebehaviour of thevariation, as a function of frequency, in'the efficiencyof a klystron improved in accordance with the invention FIG. 4 is ahighly schematic representationof anembodiment of part of a klystron inaccordance with the invention.

FIG. 1 illustrates the behaviour of a curveplottingefficiency r as afunction of frequency F, in aconventional klystron power amplifier. Weare concerned'here with a klystron having a wide operating band Band' ahigh power output. The efficiency of this klystron or, to put it anotherway, its fundamental beam current at the output circuit, is a maximum atthe centre Fo of the frequency band B.

In accordance with a prior art'technique, referred to hereinbefore, thisefficiency r is brought to a maximum through the band B, by utilisinginthe amplifier section of the klystron, cavities tuned. to frequencieshigher than the frequency Fo.

If frequency shifts of this kind, in the last two-or three cavities ofthe amplifier section, make'it possible to obtain relatively constantefficiency curves, such as that of FIG. 1, in high-power klystrons, thisis certainly not he case with klystrons of lower power.

FIG. 2 illustrates the behaviour of the efficiency curve in this kind oflower power klystron, the klystron having a passband B identical to thatof FIG. 1. Although it is possible, in this case, to obtain at thecentre F,, of the band, an efficiency r which is quite high, as in thecase of FIG. 1, it is not possible to maintain this efficiency at valuesanywhere near r in the remainder of the band.

FIG. 3 illustrates the behaviour of the efficiency curve r as a functionof frequency F, in a klystron improved in accordance with the invention,FIG. 4 schematically illustrating a sectional view of such a klystron.

The klystron shown in FIG. 4 contains an evacuated enclosure 1 ofapproximately cylindrical general shape, this enclosure exhibiting atone end an electron-gun symbolically represented by the emissive cathode2, and at the other end a collector electrode 3. The beam of electronsemitted by the cathode 2 and directed towards the collector 3, passesthrough the drift tube formed by the succession of drift spaces whereelectron bunching takes place and between which there are interposedresonators which constitute the different cavities 4 to 10.

The first cavity, through a coupler device schematically marked 11,receives the high-frequency signal for amplification. This signal,transposed to the electron beam emitted by the cathode and modulated inthe input cavity 4, is preamplified in known fashion in the firstsection I of the klystron, where the gain is developed. In the figure,this first section comprises two socalled fundamental" cavities, that isto say which are tuned within the operating frequency band B of theklystron this section, in a manner known per se, can comprise more thantwo such cavities.

The third section III of the klystron, schematically represented here bya fundamental" output cavity, with which there is associated a couplerdevice 12 picking off the amplified HF signal, is the section in whichthe HF power is extracted and, as stated hereinbefore, is in factconstituted in a manner known per se, by any suitable system forextracting the maximum power within the working frequency band, forexample two coupled cavities, a cavity associated with a filter, aresonant spiral, etc.

The second section II, the power amplifier proper, here comprises fourcavities 6, 7, 8, 9, one of which is a harmonic" cavity 7.

It goes without saying that this arrangement is purely an example andcould differ both in terms of the total number of cavities, in terms oftheir tuned frequencies (and consequently their dimensions), and interms of their arrangement along the length of the drift tube.

In all the embodiments, a klystron in accordance with the inventioncomprises, in its amplifier section II, on the one hand cavities tunedto frequencies higher than the maximum frequency F F 8/2 of the workingband, and on the other hand harmonic cavities tuned to frequencies closeto twice the minimum band frequency, namely 2F 2 (F 8/2) 2F B.

The diameter d and length l of these harmonic cavities, are respectivelyequal substantially to half the diameter D and half the length L, of thefundamental cavities.

To obtain an efficiency curve of the kind shown in full line in FIG. 3,the cavities tuned to the frequencies higher than the mean frequency Fare so tuned in such a way that the efficiency curve which would beobtained in the absence of the harmonic cavities (curve ABC), is notsymmetrical in relation to F,,, but has its peak M located between F,and the maximum frequency F of the band B. This result is achieved bytuning these cavities to frequencies higher than those which wouldproduce a curve symmetrical in relation to F in the manner adopted inconventional klystrons.

As far as the harmonic cavities are concerned, these are tuned to alittle below the frequency 2R, B, in order to reinforce the efficiencyof the klystron between the minimum frequency F, of the band B and thefrequency F The effect of these harmonic cavities upon the section AB ofthe curve of FIG. 3, is such that the efficiency curve of the thusimproved klystrons, has the form of the curve DBC.

To obtain this kind of result, a variety of embodiments are possible theklystron can have one or more harmonic cavities and one or more cavitiestuned to frequencies higher than F,,. In addition, the location of thesecavities in relation to one another, in the amplifier section II, canvary.

In the example illustrated in FIG. 4, the amplifier section II comprisesa fundamental cavity 6 tuned in the working frequency band B, then aharmonic cavity 7 tuned to a frequency slight lower than (2 F B), 1.88 Ffor example for a bandwidth B equal to 10 percent of F,,, then twocavities 8 and 9 tuned to frequencies higher than the frequency F,,; thecavity 8 is for example tuned to 1.05 F, and the cavity 9 to 1.07 F,,,it being understood of course that these values are given purely by wayof example.

Thus, assuming that the outputs of the klystrons in question are in allcases taken to an optimised load, and if we consider klystrons operatingin the S band (from 1,500 MC/s to 5,200 MC/s), with bandwidths B equalto 10 percent of the centre frequency F these klystrons producing outputpowers peaking at around 200 kW, then klystrons in accordance with theinvention will have efficiencies varying for example between 50 andpercent for the whole of the bandwidth B prior art klystrons operatingunder the same conditions of power, frequency and bandwidth, will haveefficiency variations ranging from 44 to 60 percent.

We claim:

1. A klystron for power amplification of a high frequency signal, whichklystron can be operated within a wide frequency band B centred on afrequency Fo, comprising, within an evacuated enclosure, an electron-gunemitting an electron beam, a first set of resonant cavities known as thepreamplifier" section, a second set of resonant cavities known as theamplifier section, a high frequency circuit for picking off theamplified high frequency signal and a collector, at least one of theresonant cavities in the said amplifier section being tuned to afrequency which is higher than the maximum frequency band B such thatthe frequency of the high frequency signal, for which the klystron has amaximum efficiency, is higher than the centre frequency F0 and lowerthan said maximum frequency F and said amplifier" section furthermorecomprising at least one harmonic" cavity tuned to a so-called harmonicfrequency close to a frequency (2 Fo-B) twice the minimum frequency ofthe band B, said so-called harmonic frequency being less than said 2Fo-Bfrequency by an amount such that an increase in the klystron efficiencyis achieved by this cavity in respect of high fequency signals having afrequency less least one cavity of the amplifier section which is thanthe centre frequency F and greater than the minituned to a frequencyhigher than the centre frequency mum frequency F of the frequency bandB. F0 is the last cavity of said amplifier section.

2. A klystron according to claim 1 wherein the at

1. A klystron for power amplification of a high frequency signal, which klystron can be operated within a wide frequency band B centred on a frequency Fo, comprising, within an evacuated enclosure, an electron-gun emitting an electron beam, a first set of resonant cavities known as the ''''preamplifier'''' section, a second set of resonant cavities known as the ''''amplifier'''' section, a high frequency circuit for picking off the amplified high frequency signal and a collector, at least one of the resonant cavities in the said ''''amplifier'''' section being tuned to a frequency which is higher than the maximum frequency band B such that the frequency of the high frequency signal, for which the klystron has a maximum efficiency, is higher than the centre frequency Fo and lower than said maximum frequency F2, and said ''''amplifier'''' section furthermore comprising at least one ''''harmonic'''' cavity tuned to a so-called harmonic frequency close to a frequency (2 Fo-B) twice the minimum frequency of the band B, said so-called harmonic frequency being less than said 2Fo-B frequency by an amount such that an increase in the klystron efficiency is achieved by this cavity in respect of high fequency signals having a frequency less than the centre frequency Fo and greater than the minimum frequency F1 of the frequency band B.
 2. A klystron according to claim 1 wherein the at least one cavity of the ''''amplifier'''' section which is tuned to a frequency higher than the centre frequency Fo is the last cavity of said ''''amplifier'''' section. 